Optical recording medium

ABSTRACT

There is provided an optical recording medium capable of preventing cross erase and increasing its recording density. The optical recording medium includes: a reflecting film; a first transparent film provided on the reflecting film; a first semitransparent film provided on the first transparent film; a second transparent film provided on the first semitransparent film; a recording film provided on the second transparent film, the recording film being capable of reversibly changing an atomic arrangement; and a third transparent film provided on the recording film. The first semitransparent film has a complex refractive index of n−k satisfying relationships of 0&lt;n&lt;1 and 1&lt;k, and a product of a thickness d (nm) of the first semitransparent film and an extinction coefficient k of the complex refractive index is d×k≦44.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2002-86297 filed on Mar. 26,2002 in Japan, the entire contents of which are incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to an optical recording mediumfor reversibly changing a state by irradiation with light beams torecord information. More specifically,the invention relates to a phasechange optical recording medium which has a change of state wherein theatomic arrangement of a thin film for holding recording changes betweenamorphous and crystalline arrangements.

2. Related Art

A typical phase change optical recording film has an amorphous atomicarrangement when its portion heated to its melting point or higher tomelt is rapidly cooled. When the phase change optical recording film isheld in a crystallized temperature range below the melting point for apredetermined time or longer, it is crystallized if its initial state isamorphous, whereas it remains crystal if its initial state is crystal.Since the intensity of reflected light from an amorphous region isdifferent from the intensity of reflected light from a crystallineregion, the principle of a phase change optical recording medium is thatthe intensity of reflected light is converted into the intensity of anelectric signal to be analog-to-digital converted to read information.

There are two methods for increasing the amount of information capableof being recorded in a single recording medium, i.e. a recordingcapacity. One method is a method for scaling down the pitch of recordingmarks in track directions. In this method, if the degree of scale downproceeds, the pitch becomes smaller than the size of a reproducing lightbeam, so that there are some cases where two recorded marks aretemporarily included in a reproducing beam spot. If the recording marksare sufficiently spaced from each other, a regenerative signal can begreatly modulated to obtain a signal having a large amplitude. However,if the recording marks are close to each other, the amplitude of thesignal is small, so that errors are easy to occur when the signal isconverted into digital data.

Another method for improving a recording density is a method fornarrowing the track pitch. This method can increase the recordingdensity without being greatly influenced by the decrease of the signalstrength due to the scale down of the mark pitch. However, in thismethod, there is a problem in that, in a region in which a track pitchis substantially equal to or smaller than the size of a light beam,there is caused a so-called cross erase wherein information on a certaintrack deteriorates when a writing or erasing operation is carried out inan adjacent track.

Two causes of cross erase are considered. One cause is that, whenadjacent tracks are irradiated with a beam, the light intensity of thebeam at the bottom edge thereof within a subject track is not small, sothat the recording mark of the track is deteriorated by only the effectof irradiation with light. The other cause is that, when adjacent tracksare heated by a light beam, generated heat is transferred to the tracksby heat transfer in film in-plane directions, so that the shape of arecording mark is deteriorated by the influence thereof. Since theinfluence of the cross erase due to the latter influence can be reducedby decreasing heat transfer in the film in-plane directions, it has beendevised to reduce the cross erase by more greatly promoting heatconduction in directions perpendicular to the plane of a recording filmthan in the in-plane directions by forming a so-called rapid-coolingstructure by arranging a film having a large conductivity and/or heatcapacity in the vicinity of a recording film.

For example, a conventional phase change optical recording medium shownin FIG. 10 comprises a substrate 301, a metal reflecting film 302 formedon the substrate 301, a transparent dielectric film 303 formed on themetal reflecting film 302, a recording film 304 formed on the dielectricfilm 303, a transparent dielectric film formed on the recording film304, and a cover layer 306 formed on the dielectric film 305. That is,the dielectric film 303 is arranged between the recording film 304 andthe metal reflecting film 302 for ensuring a signal strength by thereflection of light, and the dielectric film 303 is formed so as to berelatively thin, so that heat generated by the recording film 304 caneasily escape to prevent heat from being transferred in the filmin-plane direction. As the thickness of the dielectric film 303decreases, heat transfer in direction perpendicular to the plane of thefilm can be promoted to improve cross erase.

However, if the dielectric film 303 is too thin, heat transfer to thereflecting film 302 starts simultaneously with heating due to laserbeams during recording, so that there is a problem in that thetemperature rise of the recording film 304 is insufficient, whereby thetemperature of an area required to form a recording mark does not reachthe melting point. In addition, if a laser power of erase level isapplied, the mark cools immediately after heating. Therefore, thetemperature of the mark can not be held in a temperature range capableof crystallizing the mark for a sufficiently long time, so that there isa problem in that it is difficult to crystallize the mark, i.e. to carryout an erasing operation, thereby remarkably deteriorating the erasingrate.

Conversely, if the dielectric film 303 is too thick, there is no problemon the power margin and erasing rate with respect to laser beams duringrecording. However, as described above, heat transfer into the plane ofthe film is not only promoted to violently cause cross erase, but thecooling rate of the recording film 304 is slow. Therefore, there is aproblem in that the region melt during recording is crystallized againwithout being amorphous, so that the formed mark is too small.

Japanese Patent Laid-Open No. 2000-215516 discloses that cross erase canbe suppressed by including at least a recording layer, a top protectivelayer, an intermediate layer and a reflecting layer in order from alight incident side and by defining characteristics of the materials ofthe intermediate and reflecting layers. However, since this method doesnot sufficiently select the material of the intermediate layer andselects a material having a low thermal conductivity, it is difficult tocarry out rapid cooling, so that this method can not sufficiently reducecross erase.

Thus, the thickness and heat conduction characteristics of thedielectric film between the metal reflecting film and the recording filmare required to simultaneously eliminate the problems on the powersensitivity in recording, cross erase, recrystallization and erasingrate. However, conventional means can not simultaneously satisfy all ofthem.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to eliminate theaforementioned problems and to provide an optical recording medium whichis capable of preventing cross erase and which has a high recordingdensity.

According to a first aspect of the present invention, an opticalrecording medium includes: a reflecting film; a first transparent filmprovided on the reflecting film; a first semitransparent film providedon the first transparent film; a second transparent film provided on thefirst semitransparent film; a recording film provided on the secondtransparent film, the recording film being capable of reversiblychanging an atomic arrangement; and a third transparent film provided onthe recording film, wherein the first semitransparent film has a complexrefractive index of n−ik satisfying relationships of 0<n<1 and 1<k, anda product of a thickness d (nm) of the first semitransparent film and anextinction coefficient k of the complex refractive index is d×k≦44.

According to a second aspect of the present invention, an opticalrecording medium includes: a reflecting film; a first transparent filmprovided on the reflecting film; a first semitransparent film providedon the first transparent film; a second transparent film provided on thefirst semitransparent film; a recording film provided on the secondtransparent film, the recording film being capable of reversiblychanging an atomic arrangement; and a third transparent film provided onthe recording film, wherein the first semitransparent film is formed ofa simple substance (elementary substance) or alloy containing 90 at % ormore of at least one of aluminum, silver, gold and copper, the firstsemitransparent film having a thickness of 25 nm or less.

According to a third aspect of the present invention, an opticalrecording medium includes: a reflecting film; a first transparent filmprovided on the reflecting film; a first semitransparent film providedon the first transparent film; a second transparent film provided on thefirst semitransparent film; a recording film provided on the secondtransparent film, the recording film being capable of reversiblychanging an atomic arrangement; a third transparent film provided on therecording film; and a second semitransparent film provided on the thirdtransparent film, wherein the first semitransparent film is formed of asimple substance (elementary substance) or alloy containing 90 at % ormore of at least one of aluminum, silver, gold and copper, the firstsemitransparent film having a thickness of 25 nm or less, and whereinthe second semitransparent film is formed of a simple substance or alloycontaining 90 at % or more of at least one of aluminum and silver, thesecond semitransparent film having a thickness of 25 nm or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing the construction of the firstembodiment of a phase change optical recording medium according to thepresent invention;

FIG. 2 is a timing chart showing the variation in power of a laser pulsewith the elapse of time;

FIG. 3 is a graph showing a temperature history in the outer edgeportion of a mark when an optical recording medium in the firstembodiment and a conventional recording medium are irradiated with alaser pulse of a recording power;

FIG. 4 is a graph showing a temperature history in the outer edgeportion of an adjacent track mark when an optical recording medium inthe first embodiment and a conventional recording medium are irradiatedwith a laser pulse of a recording power;

FIG. 5 is a graph showing the variation in jitter value after tenoverwrite operations with respect to the distance between asemitransparent film and a recording film in the first embodiment;

FIG. 6 is a graph showing the variation in jitter value after tenoverwrite operations with respect to the thickness of a semitransparentfilm in the first embodiment;

FIG. 7 is a table showing constructions with respect to variousmaterials of a dielectric film adjacent to a recording film in the firstembodiment;

FIG. 8 is a graph showing the variation in jitter values after tenoverwrite operations with respect to the product of the thickness of areflecting film and an extinction coefficient in the first embodiment;

FIG. 9 is a sectional view showing the construction of the secondembodiment of a phase change optical recording medium according to thepresent invention;

FIG. 10 is a sectional view showing the construction of a conventionalphase change optical recording medium;

FIG. 11 is an illustration showing a point which is equal to theoutermost edge of a recording mark of a recording film;

FIG. 12 is an illustration showing the position of a beam spot after arecording power irradiation starts;

FIG. 13 is a table showing the measured results of jitter values inExamples 1 and 2 of an optical recording medium in the first embodiment;

FIG. 14 is a table showing the measured results of jitter values inExample 3 of an optical recording medium in the first embodiment;

FIG. 15 is a table showing the measured results of jitter values inExamples 4 and 5 of an optical recording medium in the first embodiment;

FIG. 16 is a table showing the measured results of complex refractiveindexes in Examples 1, 3 and 4 of an optical recording medium in thefirst embodiment when the wavelength of a light source is 650 nm; and

FIG. 17 is a table showing the measured results of jitter values inExamples 1 and 2 of an optical recording medium in the secondembodiment.

DESCRIPTION OF THE EMBODIMENTS

Referring now to the accompanying drawings, the embodiments of thepresent invention will be described below. The inventors havesufficiently considered the above-described objects and have concludedthat the ideal construction of a single layer recording medium satisfiesthe following requirements.

(1) Since heat conduction up to 10 ns after irradiation with a recordingbeam should be such that it allows heating of the recording film to themelting point or more during recording at an actual laser power, thethermal conductivity of a first dielectric film close to a recordingfilm must not be too high.

(2) Since heat conduction from 10 ns till 30 ns should surely makeamorphous during solidification, a second film of a sufficiently highthermal conductivity must be provided right above the first dielectricfilm.

(3) Since heat conduction should thereafter prevent the occurrence ofcross erase due to heat transfer to an adjacent track, a third filmhaving a high heat capacity must carry thermal diffusion.

As an example of a concrete construction, it is required that a film (alow heat conductivity film) having a heat conductivity, which is equalto or lower than that of ZnS:SiO₂ (mole ratio ZnS:SiO₂=80:20), andhaving a thickness of 6 nm or more is provided as each of dielectricfilms for vertically sandwiching a recording film in order to ensurerecording sensitivity. A film having a heat conductivity, which is equalto or higher than ZnS:SiO₂, may be provided between the recording filmand a reflecting film. For example, an interface film for promotingcrystallization may be provided just above the recording film. Thethickness of the film must not exceed 10 nm if the film contacts therecording film. As the next condition, the thickness of the low heatconductivity film must not be 20 nm or more. If the thickness is 20 nmor more, the time for the recording film to be held at a hightemperature is too long, so that heat conduction in lateral directionsare more greatly promoted than heat conduction in directionsperpendicular to the reflecting film, thereby violently causing crosserase.

In order to meet the requirement (2), it is advantageous that a secondfilm having a high heat conductivity is selected regardless ofdielectric and metal, and it was revealed that a metal having a heatconductivity ten times or more as high as that of a dielectric isadvantageous after various studies. Moreover, it was revealed that thereare some cases where the reflectance of the recording medium can not beset to be in a preferred range if a metal film is used as the secondfilm.

In the case of a so-called High-to-Low polarity, which means that Rc>Rawhere the reflectance of a non-grooved crystal portion is Rc and thereflectance of a non-grooved amorphous portion is Ra, the greater Rc−Ralead to the larger signal amplitude. At the same time, for the sameamount of Rc−Ra, the smaller Ra is preferred because it leads to lessnoise. Assuming that the optical constant (complex refractive index) ofthe material of a preferred second film to satisfy this was found to be0<n<1 and k>1 where n−ik is the refractive index of the second film.

As another requirement satisfying the above described request, it wasalso revealed that a fourth transparent film between the second andthird films must be provided so that the second film does not contactthe third film. Therefore, it was revealed that the above describedrequests (1) to (3) can be satisfied by adopting a suitable metal filmas the second film, a transparent dielectric film as the fourth film,and a metal reflecting film having a high heat capacity as the thirdfilm.

(First Embodiment)

The construction of the first embodiment of a phase change opticalrecording medium according to the present invention is shown in FIG. 1.This phase change optical recording medium in the first embodimentsatisfies the above described requests (1) to (3), and comprises asubstrate 101 of, e.g., polycarbonate, a metal reflecting film 102formed on the substrate 101, a transparent dielectric film 103 formed onthe metal reflecting film 102, a semitransparent metal film 104 formedon the dielectric film 103, a transparent dielectric film 105 formed onthe semitransparent metal film 104, a recording film formed on thedielectric film 105, a transparent dielectric film 107 formed on therecording film 106, and a cover layer 108 formed on the dielectric film107.

This phase change optical recording medium in the first embodiment canreduce cross erase. Referring to the drawings, this will be described indetail. With respect to characteristics of the recording film 106 foruse in this embodiment, it is assumed that the melting point is 600° C.and the crystallization starting temperature is 180° C. It is alsoassumed that the recording laser power emits light in timing shown inFIG. 2. That is, it is assumed that there are given powers of an eraselevel Perase for a period of t<0, a record level Pwrite for 0<t<t₀, aread level Pread for a period of t₀<t<t₁ (this period will behereinafter referred to as an off pulse), and an erase level Perase int₁<t. FIG. 3 shows a temperature history of a recording film at a point(which will be hereinafter referred to as a point P), at which acoordinate position in track directions at t=0 is equal to the center ofa spot of a laser beam and at which a position in radial directions ofan optical disk is equal to the outermost edge of a recording mark. Theposition of a point is shown in FIG. 11. In FIG. 3, a graph g₁ shows atemperature history of a recording film 106 of a phase change opticalrecording medium in this embodiment, and a graph g₂ shows a temperaturehistory of a recording film 304 of a conventional phase change opticalrecording medium shown in FIG. 10. Since this point P is positioned onthe outermost edge of a range to be a mark, a portion inside the pointP, i.e. a place closer to the center of a track, is heated to melt. Aportion outside the point P, i.e. a place apart from the center of thetrack, does not melt since its temperature does not reach the meltingpoint. In general, it is considered that it is determined whether aphase change optical recording medium is recrystallized after melting,by examining a temperature history at the point P to determine whetherthe point P is held for a predetermined time or longer in a temperaturerange which is the melting point or lower and which is a crystallizingtemperature or higher. This predetermined time is called a phase changeoptical recording crystallizing time which is a physical value of a usedrecording film and which is a value determined by the composition of therecording film. Since the cooling rate of the recording film inside thepoint P after melting is higher than the cooling rate at the point P,the time in which the point inside the point P is held in the abovedescribed temperature range is shorter than the crystallizing time ifthe time in which the point P is held in the above-described temperaturerange is shorter than the crystallizing time. Therefore, in this case,crystallization does not occur over the whole range in which therecording mark is formed.

In FIG. 3, it is forced to be heated by a recording power up to t₀, andits temperature substantially reaches the melting point. Thereafter, itis rapidly cooled by a cooling pulse. In this embodiment, thetemperature after the cooling pulse is completed, i.e. the temperatureat t₁, is 300° C. (see graph g₁). Subsequently, since it is irradiatedwith a laser beam of an erase level, the cooling rate is decreased byits influence, but it is continuously cooled. Since the temperaturereaches a crystallizing temperature t_(x) at time t₂, crystallizationdoes not subsequently occur. Therefore, if the recording film 106 of amaterial having a crystallizing time longer than ΔT_(a)=t₂−t₁ is used,it is possible to prevent the recrystallization of the recording mark.

On the other hand, if the conventional construction is used, the coolingrate during the cooling pulse is slow as shown by the graph g₂ in FIG.3, the temperature is only decreased to 400° C. even at t₁ after thecooling pulse is completed. Since the subsequent cooling is carried outfrom 400° C., the cooling time is longer than that in this embodiment.In a conventional example, the time at the crystallizing temperaturet_(x) is a time t₃, and the recording film is held at a temperature, atwhich crystallization can be carried out, for a long time of t₃−t₀.Therefore, if a recording film having a crystallizing time ofΔt_(b)=t₃−t₀ or shorter is used, recrystallization occurs.

As compared with a phase change optical recording medium in thisembodiment, Δt_(a)<Δt_(b), so that the phase change optical recordingmedium in this embodiment is advantageous in order to preventrecrystallization. In other words, it is found that, in this embodiment,the width of a melting region required to form a recording mark havingthe same width may be narrower, so that thermal effect to a mark rowrecorded in an adjacent track is small.

As described above, in the medium in this embodiment, the cooling rateis particularly high mainly for t₀<t<t₁, i.e. for the period of thecooling pulse. The reason for this is that the distance from the metalfilm closest to the recording film 106 is shorter than that in theconventional phase change optical recording medium. The metal filmclosest to the recording film 106 is herein the semitransparent metalfilm 104 shown in FIG. 1. Since it is semitransparent, it is a very thinfilm. However, since its heat conductivity is far greater than that of adielectric or semiconductor, it can enhance the cooling rate during thecooling pulse. Since the heat flow during the period of cooling pulseoccurs for a very short time immediately after a recording powerirradiation, the temperature does not yet rise even if the metal film104 is very thin and thus have a very small heat capacity. Therefore,the heat flow in the dielectric films 105 and 107 adjacent to therecording film 106 is controlled by a direction perpendicular to therecording film 106.

It is herein compared with a case where the distance between therecording film 304 and the metal reflecting film 302 decreases withoutchanging the construction of the conventional phase change opticalrecording medium shown in FIG. 10. In the conventional phase changeoptical recording medium, if the metal reflecting film 302 approachesthe recording film 304, the heat conductivity is too high because of thethick metal reflecting film 302, so that most of heat received by laserirradiation is transmitted through the reflecting film 302 to betransferred in a film in-plane direction. That is, the laser power forreaching a temperature required to form a recording mark increases.Therefore, there is a problem in that an expensive laser is required andpower consumption is large.

Similarly, graphs g₃ and g₄ in FIG. 4 show temperature histories on theouter edge of a mark of an adjacent track (nearer to a noticed recordingtrack) with respect to the phase change optical recording medium in thisembodiment and the conventional phase change optical recording mediumshown in FIG. 10, respectively. The outer edge of the mark of theadjacent track shown by the graph is shown by a point Q in the plan viewof FIG. 12. The recording timing is the same as that in FIG. 3. Theposition of a beam spot after starting a recording power irradiation,i.e. t=0, along with the end of the recording power irradiation, i.e.,t=t₀, is shown in FIG. 12.

As described above, in the phase change optical recording medium in thisembodiment, the heat flow in the recording track for the period of t<t₁is controlled by a vertical component, so that heat diffusion to anadjacent track is small. Therefore, in the temperature history of thephase change optical recording medium in this embodiment, thetemperature after the irradiation with the recording pulse is a lowtemperature of 270° C. as shown by the graph g₃.

On the other hand, in the conventional phase change optical recordingmedium shown in FIG. 10, the dielectric films 303 and 305 adjacent tothe recording film 304 are relatively thick, heat transfer in lateraldirections occurs to some extent. Therefore, the maximum temperature inthe outer edge portion of the adjacent track is a relatively hightemperature of 350° C. as shown by the graph g₄.

After the heat history of the outer edge portion of the adjacent trackis studied to compare the time held in a temperature range at whichcrystallization can be carried out, Δt_(c)=t₆−t₅ in this embodiment,Δt_(d)=t₇−t₄ in the conventional example, and Δt_(c)<Δt_(d), so thatcross erase can be suppressed in this embodiment.

A publicly known example which is similar to this embodiment at firstsight will be briefly described below. Japanese Patent Laid-Open No.2000-222777 discloses that it is possible to improve the cooling powerof a recording film to reduce overwrite distortion by providing arecording layer, a reflecting layer allowing transmission of lightbeams, and a thermal diffusion layer on the far side from the reflectinglayer viewed from a light incident surface. However, as clearlydescribed in the above described Japanese Patent Laid-Open No.2000-222777, this construction is provided for enhancing density bysetting the whole transmittance to be 50 to 70%, providing a secondrecording layer on the far side from the light incident surface withrespect to the medium, and using two layers of recording media, so thatreflectance decreases due to the enhancement of transmittance.Therefore, as compared with a recording medium designed to holdrecording by a single layer having a high reflectance, the difference inreflectance between crystal and amorphous is small, the amplitude of aregenerative signal is small, and jitter is large. That is, in order toobtain the same signal quantity, a track pitch or mark pitch forrecording must be set to be larger than that of a single layer medium,so that the recording capacity per one layer of recording film issubstantially smaller than that of the single layer medium.

On the other hand, this embodiment is limited to a single layer typerecording medium which is not required to enhance transmittance as awhole, and is designed to comply with a request for the increase of therecording capacity by increasing the recording density of the singlelayer recording medium without increasing the recording capacity bydouble layers. Therefore, the medium using the single layer is notrequired to enhance the whole transmittance unlike the above describedJapanese Patent Laid-Open No. 2000-222777, and it is important toincrease the difference in reflectance between crystal and amorphous asdescribed above. In addition, there is an advantage in that the mediumof the single layer can be far easily produced than a medium of two ormore layers since it is not required to carry out complicated steps ofpreparing a thin separating layer and aligning two layers.

In the above-described Japanese Patent Laid-Open No. 2000-222777, thereflecting layer must be semitransparent to ensure transmittance inorder to realize a laminated medium using recording films of two or morelayers. As clearly described in the above-described Japanese PatentLaid-Open No. 2000-222777, cooling degree is insufficient as a sideeffect thereof, and in order to compensate for this, a dielectric filmhaving a high thermal conductivity is added.

On the other hand, the idea of this embodiment has been made, in asingle layer medium in which high transmittance is not required, inorder to reconsider thermal conduction from a standpoint of reduction ofcross erase and in order to freely control cooling processes in threestages.

The semitransparent reflecting layer in the above-described JapanesePatent Laid-Open No. 2000-222777 is substituted for the conventionaltotal reflection type reflecting layer. The semitransparent layer 104 inthis embodiment serves to carry out the cooling process in the secondstage of the cooling processes in three stages. Therefore, it isdifficult to make the idea of the present invention even after the abovedescribed Japanese Patent Laid-Open No. 2000-222777 is known, so that itis clear that the present invention sufficiently has novelty andinventive step over the above described Japanese Patent Laid-Open No.2000-222777.

The cooling processes in three stages are as follows. First, the firstcooling process is a cooling process occurring before to in FIG. 3, i.e.for a very short time immediately after a recording power irradiation.According to the construction in this embodiment, the first coolingprocess is mainly carried out by the dielectric film 105 close to therecording film. Since the heat conductivity of the dielectric film 105is sufficiently low, its temperature easily reaches its melting point,there is no side effect for decreasing sensitivity. The second coolingprocess is a dominant cooling mechanism for a period of t₀ to t₁ shownin FIG. 3, and is carried out by the semitransparent metal film 104.Since this semitransparent metal film 104 has a far higher heatconductivity than dielectric films although it is very thin, it has thefunctions of promoting rapid cooling and heat transfer in directionsperpendicular to the plane of the film and of inhibiting heat transferin lateral directions, which causes cross erase. The third coolingprocess is a cooling mechanism acts mainly after t₁ in FIG. 3, and ismainly carried out by the reflecting film 102 and dielectric film 103 onthe side of the reflecting film. This has the function of backing up thecooling of the semitransparent metal film 104 by utilizing the high heatconductivity and thermal capacity due to the thick reflecting film,similar to the recording medium having the same structure as that in theconventional example.

Of course, with respect to the inhibition of cross erase, the effects ofthe present invention has an inventive step over the technique disclosedin the above described Japanese Patent Laid-Open No. 2000-222777. Thiswill be described below. The construction disclosed in the abovedescribed Japanese Patent Laid-Open No. 2000-222777 can more surelypromote heat flow in vertical directions to more effectively inhibitcross erase than the semitransparent medium having no thermal diffusionlayer. However, it is not sufficient since the heat conductivity of thethermal diffusion layer provided on the far side from the reflectingfilm is about one tenth as small as metals. In particular, there is aproblem in that heat transfer in film in-plane directions continues forten to tens ns after a beam irradiation, to promote the occurrence ofcross erase due to heat conduction. In addition, since the reflectinglayer is provided just above the interface layer, heat violently escapesduring and immediately after a laser beam irradiation, so that there isa problem in that it is required to provide high power to heat it to atemperature above the melting point, which is necessary for recording.

Referring to FIG. 1, an example of this embodiment will be describedbelow in detail.

FIG. 1 shows a sectional construction of an example of an opticalrecording medium in this embodiment. The optical recording medium inthis example has a construction wherein a reflecting film 102 of an Agalloy, a dielectric film 103 of ZnS:SiO₂, a semitransparent metal film104 of an Ag alloy, a dielectric film 105 of ZnS:SiO₂, a recording film106 of GeSbTe, and a dielectric film 107 of ZnS:SiO₂ are stacked inorder from the opposite side to light incidence. The thickness of thesefilms is 100 nm, 10 nm, 15 nm, 10 nm, 12 nm and 135 nm, respectively, inthe same order. A single layer film of an Ag alloy was separatelyprepared, and its complex refractive index (optical constant) wasmeasured by ellipsometry. As a result, the complex refractive index was0.50-1.73i at a wavelength of 405 nm. Similarly, the complex refractiveindex (optical constant) of GeSbTe being the recording film 106 wasmeasured. As a result, the complex refractive index was 2.6-2.1i in anamorphous state, and 1.45-2.8i in a crystal state. In addition, theabove described ZnS:SiO₂ is ZnS:SiO₂=80:20 in mole ratio. The opticalrecording media in this example was prepared by depositing theabove-described films on a polycarbonate substrate 101 having athickness of 1.1 mm in the above-described order by the sputteringmethod. After deposition, an ultraviolet curing resin was spin-coated tobe photo-cured to form a cover layer 108 having a thickness of 0.1 mm.

Then, the whole surface of the recording film 106 was crystallized by aphase change medium initializing system having an elliptical beam havinga width of 50 μm and a length of 1 μm, to complete the medium. Grooveshaving a depth of 40 nm are provided on the substrate 101 at intervalsof 0.6 μm. When it is evaluated as a land groove recording, theabove-mentioned substrate yields the track pitch of 0.3 μm. Hereinafter,the groove track means a track farther from the light incident surface,and the land track means a track nearer to the light incident surface.

Using the above described optical recording medium, record and eraseexperiments were carried out. Evaluation was carried out by means of anoptical disk evaluating system having a pick-up which has an objectivelens of NA=0.85 and a semiconductor laser having a wavelength of 405 nm.Assuming that a pattern having a shortest mark length of 0.15 μm was 3Tand a pattern having a longest mark length of 0.55 μm was 11T, arecording experiment was carried out by irradiating with pulse patternsformed by combining a series of 3, 4, . . . , 11T at random.

The optimum recording power was found out to be used to carry out therecording method.

As a comparison, media of the same film material were trial-manufacturedby changing the distance (which is assumed to be d₀) between thesemitransparent metal film 104 and the recording film 106, andevaluated. The distance do denotes the shortest distance between thebottom face of the semitransparent metal film 104 and the top face ofthe recording film 106 in FIG. 1. The results of this evaluation isshown in FIG. 5. The jitter value (curve k₁ in the figure) after tenoverwrite operations in only the corresponding track was sensitivelydeteriorated when d₀ was less than 5 nm and exceeded 15 nm. This iscaused by the fact that the recording sensitivity is insufficient whend₀ is decreased and that an amorphous mark is recrystallized to bereduced when d₀ is increased since the time held in a crystallizingtemperature range is too long. The jitter value (curve k2 in the figure)after overwriting the same pattern in both adjacent tracks ten times wasdeteriorated when d₀ was large. This is caused by the fact that theouter edge portion of the mark in the corresponding track was partiallycrystallized so that the width of the mark was thin. The rate ofincrease of jitter before and after recording in the adjacent track wasparticularly bad when do was large. Also from this point, d₀ is notpreferably too thick.

By the way, while do has been changed in FIG. 1, if the thickness of onefilm is separately changed, its reflectance is simultaneously changed,so that there are some cases where it is not possible to obtain areflectance suitable for a recording medium. Therefore, the variation inreflectance was corrected by changing the thickness of the ZnS:SiO₂ filmnearest to the light incidence in FIG. 5, i.e. the thickness of thedielectric film 107 in FIG. 1, so that any media can obtain the samereflectance. Since the dielectric film 107 is originally sufficientlythick, even if the thickness varies in a thickness adjusting range(about ±20 nm) for purposes of correction of the variation inreflectance, thermal function does not vary, and the cooling rate of therecording film does not vary. Meanwhile, in the following descriptions,in experiments for examining dependency on thickness for the samereason, the dielectric film on the side of light incidence (thedielectric film 107 in FIG. 1) in addition to the thickness of the filmto be noticed was simultaneously varied to prevent the difference inreflectance from varying.

In this example, the extinction coefficient k of the Ag alloy of thesemitransparent film is 1.73, and the thickness d of the semitransparentmetal film 104 is 15 nm, so that its product d×k=25.95.

For a comparison, a medium having a different d×k value wastrial-manufactured. In this case, the medium has a film constructionwherein a reflecting film of an Ag alloy, a dielectric film of ZnS:SiO₂,a semitransparent film of an Ag alloy, a dielectric film of ZnS:SiO₂, arecording film of GeSbTe and a dielectric film of ZnS SiO₂ are stackedin order from the opposite side to the light incidence. At this time,the thickness d of the semitransparent film was varied. As shown in FIG.6, the results are shown by a graph k₄ when the thickness d of thesemitransparent film exceeds 25 nm. From characteristics of the jittervalue after overwriting the same pattern in an adjacent track ten times,the jitter value exceeds 10% and rapidly increases. It was found thatthis was not preferred. The thickness d=25 nm of the semitransparentfilm corresponds to d×k=44 since the complex refractive index (opticalconstant) is 0.50-1.73i at a wavelength of 405 nm as described-above.The reason why the jitter value increases to deteriorate characteristicsif the thickness of the semitransparent film exceeds 25 nm is that thesilver alloy forming the semitransparent film is too thick and beyondthe semitransparent region in this embodiment, so that thesemitransparent film is not different from the usual total reflectionrecording medium. Therefore, the product d×k of the thickness d (nm) andextinction coefficient of the semitransparent film is preferably 44 orless. In FIG. 6, k₃ is a graph showing characteristics of jitter valuesafter ten overwrite operations in only a corresponding track.

Then, it is assumed that the medium described in the above-describedexample is a medium 1. For comparison, there were trial-manufactured amedium (medium 2) wherein a reflecting film of an Ag alloy, a dielectricfilm of ZnS:SiO₂, a semitransparent film of an Ag alloy, a dielectricfilm of ZrO₂, a recording film of GeSbTe and a dielectric film ofZnS:SiO₂ are stacked in order from the opposite side to the lightincidence, and a medium (medium 3) wherein a reflecting film of an Agalloy, a dielectric film of ZnS:SiO₂, a semitransparent film of an Agalloy, a dielectric film of TiO₂, a recording film of GeSbTe and adielectric film of ZnS:SiO₂ are stacked in order. The constructionsthereof are shown in FIG. 7. That is, the medium 2 was formed bysubstituting the dielectric film of ZrO₂ for the dielectric film 105 ofZnS:SiO₂ in the above described example, and the medium 3 was formed bysubstituting the dielectric film of TiO₂ for the dielectric film 105 ofZnS: SiO₂ in the above described example.

In the medium 2, the jitter value was 9.8% in only one track, and thejitter value was 10.2% after writing in both adjacent tracks. Thesevalues were preferred values. However, in the medium 3, the jitter valuewas 14.4% in one track, and the jitter value was 15% or more afterwriting in an adjacent track, so that characteristics are deteriorated.After measuring the heat conductivity of the ZrO₂ and TiO₂ films, theheat conductivity of the ZrO₂ film was 1.5 W/mK, and the heatconductivity of the TiO₂ was 10 W/mK. On the other hand, the heatconductivity of ZnS:SiO₂ in this embodiment was 0.6 W/mK. Therefore, itwas revealed that the heat conductivity of the dielectric film 105sandwiched between the recording film 106 and the semitransparent metalfilm 104 must not be too high. The heat conductivity of the dielectricfilm 105 is preferably 2.0 W/mK or less.

Meanwhile, the heat conductivity of a thin film can be measured by thealternating light irradiation method, e.g. PIT-1 produced byShinku-Riko, Co., Ltd.

Of the two dielectric layers 103 and 104 sandwiched between thereflecting film 102 and the recording film 106, the materials of thedielectric film 104 suitably include SiO₂ and HfO₂, the heatconductivity of which is not too high, in addition to ZnS:SiO₂ and ZrO₂described herein. Conversely, Al₂O₃and AlN are not preferred since theirheat conductivity is too high.

Moreover, for comparison, a phase change recording medium in an example,and a phase change recording medium having a reflecting film 102 havinga different thickness from that of the phase change recording medium inthis example were prepared. As the material of the reflecting film 102,a silver alloy having an optical constant of 1.73 was used, and thethickness of the reflecting film 102 was changed. As shown in FIG. 8, ifthe product rk of the thickness r and the extinction coefficient k issmaller than 60, the jitter value suddenly deteriorates. This is causedby the deterioration of the modulation amplitude reflected lightreturning to the pick-up (the difference in reflectance between crystaland amorphous portions) to decrease the signal strength and by thedeterioration of the cooling power of the reflecting film to start tocause the recrystallization during the formation of an amorphous markand to start to cause cross erase, since the reflecting film starts totransmit light. In FIG. 8, k₅ is a graph showing characteristics ofjitter values after ten overwrite operations in only a correspondingtrack, and k₆is a graph showing characteristics of jitter values afteroverwriting the same pattern in both adjacent tracks ten times. Herein,the same material was used and the thickness was changed. When r×k issmaller than 60 even if the material is changed, the jitter valuedeteriorates. The reflecting layer 102 is not preferably formed of amaterial having a small value of k, e.g. Si or Ge.

Meanwhile, the silver alloy used for the semitransparent metal film 104in this example is AgPdCu which consists of 97.5 at % of Ag, 1.0 at % ofPd and 1.5 at % of Cu.

For comparison, various media having semitransparent films 104 havingdifferent compositions were trial-manufactured, and jitter valuesthereof were examined. The thickness of the semitransparent film 104 wasthe same as that in this example.

Referring to FIG. 13, the silver alloy will be described. As shown inFIG. 13, there were trial-manufactured a media in example 1 wherein thecomposition ratio of silver of the semitransparent film 104 was Ag=97.5at %, a media in example 2 wherein the composition ratio of silver wasAg=92 at %, and a media in comparative example 1 wherein the compositionratio of silver was Ag=88 at %. Alloy films of above-mentionedcompositions were deposited using sputtering targets having thecompositions of Ag=97.5 at %, Ag=92 at % and Ag=88 at % respectively.Single layer thin films of Ag-alloys having above-mentioned compositionswere formed by sputtering, and the complex refractive index (n−k) wasmeasured. The results of measurement are shown in FIG. 13. In FIG. 13,the wavelength of light was 405 nm. As can be seen from FIG. 13, k didnot so vary by the increase of the composition ratio of Ag, but ngreatly increased. With respect to these media in examples 1, 2 andcomparative example 1, jitter values after ten random signal recordingoperations in only a corresponding track are shown in 1Tr column of FIG.13, and jitter values after ten overwrite operations of the same patternin both adjacent tracks are shown in 3Tr column of FIG. 13. As can beseen from FIG. 13, the rates of increase of jitter values before andafter recording in the adjacent tracks are small to decrease cross erasedue to thermal effects. However, it was revealed that the jitter valuesthemselves are decreased by the increase of the composition ratio of Agand that the jitter value exceeds 13% in comparative example 1 whereinthe composition ratio of Ag is 88%, the jitter value substantiallyhaving insufficient characteristics.

The reason for this will be described below. The optical recordingmedium can obtain a high signal strength as the difference between thereflectance of the amorphous portion and the reflectance of the crystalportion increases. However, in the three kinds of media compared above,the reflectance decreases as the comparative ratio of Ag of thesemitransparent film increases. This is related to the real part of thecomplex refractive index of the semitransparent film as shown in FIG.13. In general, with respect to characteristics of the phase changeoptical recording medium, as the difference in reflectance betweenamorphous and crystal portions is greater, the signal strength ishigher, and reproduction can be carried out with smaller errors. As theimaginary part of the complex refractive index of the semitransparentfilm is larger, or as the real part thereof is smaller if the imaginarypart is the same, the difference in reflectance can be increased. Inexample 1 of FIG. 13, n=0.5 which is low. However, it increases in orderof example 2 and comparative example 1. In particular, the comparativeexample does not satisfy n<1 which is a preferred range in thisembodiment, and the jitter value is particularly bad. Therefore, it isfound that the composition ratio of Ag in the Ag alloy is preferably 90%or more.

Referring to FIG. 14, the Al alloy will be described below. There wereprepared a medium in example 3 wherein a layer of AlTi alloy containing99.0% of Al was used as the semitransparent film 104, and a medium incomparative example 2 wherein a layer of AlTi alloy containing 88.0% ofAl. With respect to the media in example 3 and comparative example 2,jitter values after ten random signal recording operations in only acorresponding track are shown in 1Tr column of FIG. 14, and jittervalues after ten overwrite operations of the same pattern in bothadjacent tracks are shown in 3Tr column of FIG. 14. As can be seen fromFIG. 14, the jitter values were good in example 3 since the compositionratio of Al is high and n is small, whereas the jitter values weredeteriorated in comparative example 4 since the composition ratio of Alis low and n is increased four times. In FIG. 14, the wavelength usedfor the measurement of the complex refractive index was 405 nm.Therefore, the composition ratio of Al in the Al alloy is preferably90.0% or more.

Referring to FIG. 15, a case where Au or Cu based metal is used as thematerial of the semitransparent film 104 will be described below. InFIG. 15, the wavelength used for the measurement of the complexrefractive index was 405 nm. Both of Au and Cu have a high heatconductivity, and are preferably used for obtaining the cooling effectsas described above. However, since Au and Cu have a greater value of nthan an Ag or Al base alloy at a wavelength of 405 nm as shown in FIG.15, they can optically obtain good effects. If Cu is used as example 5,the jitter values are better than those in example 4 wherein Au is used,but they are inferior to a case where an Ag or Al base alloy is used. Inaddition, n was suppressed to the minimum since pure metal was used forexperiments. However, if the pure metal was combined with another metalto form an alloy in order to increase life, n was further increased, andthe difference in reflectance decreased to further deteriorate jitter asdescribed above. In addition, FIG. 15 shows optical constants of Mo ascomparative example 3. Metals other than Ag, Al, Au or Cu base materialsare not preferred since n of Mo is higher.

FIG. 16 shows optical constants of materials used for thesemitransparent film 104 in examples 1, 3, 4 and 5 when the wavelengthof the light source is 650 nm. Since both of Au and Cu can obtain a lowvalue of n unlike the case of a wavelength of 405 nm, they can realize apreferred difference in reflectance as an optical recording medium, sothat good jitter values can be obtained. Then, it was found that each ofthe composition ratio of Au in the Au alloy and the composition ratio ofCu in the Cu alloy is preferably 90% or higher in order to obtain goodjitter values. Furthermore, it is found that the materials used for thesemitransparent film 104 in examples 1 and 3 are beyond the scope ofthis embodiment for the longer wavelength of 650 nm since n is higherthan that in the case of a short wavelength (405 nm).

While nothing has been provided between the recording film 106 and thedielectric films 105 and 107 sandwiching the recording film 106therebetween in this example, a thin film may be provided before andafter the recording film 106 in order to improve the erasing rate. Thematerials of this thin film include GeN, SiC, SiN and CrO. If the heatconductivity of this thin film is within the range defined in thisembodiment, i.e. below 2.0 W/mK, it is within the scope of thisembodiment, so that the above described thin film may be substituted fora part of the dielectric film 105 of FIG. 1. Even if the heatconductivity of the above described thin film exceeds 2.0 W/mK, if thethickness of the thin film is 5 nm or less, it is possible to obtain thesame effects as those in this embodiment, and the above described thinfilm can be substituted for a part of the dielectric film of FIG. 1.

In the above-described embodiment, the reflecting film 102, thedielectric film 103, the semitransparent metal film 104, the dielectricfilm 105, the recording film 106 and the dielectric film 107 have beensequentially stacked on the substrate 101 so as to allow light to beincident on the dielectric film 107. Conversely, the dielectric film107, the recording film 106, the dielectric film 105, thesemitransparent metal film 104, the dielectric film 103 and thereflecting film 102 may be sequentially stacked on a transparentsubstrate so as to allow light to be incident on the substrate. In thiscase, a light source equipped with an objective lens suited to allowlight to be incident on the substrate should be used. In addition, aprotective film for protecting the reflecting film 102 may be providedon a surface opposite to a surface on which the dielectric film 103 isformed.

As mentioned-above, the semitransparent metal film 104 may be formed ofa simple substance (elementary substance) or alloy containing 90 at % ormore of at least one of aluminum, silver, gold and copper.

(Second Embodiment)

Referring to FIG. 9, the second embodiment of a phase change opticalrecording medium according to the present invention will be describedbelow. The phase change optical recording medium in this embodiment hasa construction wherein a metal reflecting film 202, a transparentdielectric film 203, a semitransparent metal film 204, a dielectric film205, a recording film 206, a dielectric film 207, a semitransparentmetal film 208 and a cover layer 209 are sequentially stacked on asubstrate 201 of polycarbonate. That is, in the phase change opticalrecording medium in the first embodiment, another semitransparent metalfilm 208 is provided on the light incident side.

The features of this embodiment are as follows. In the conventionalconstruction shown in FIG. 10, i.e. in the construction wherein threelayers of dielectric/semitransparent film/dielectric film are providedbetween a recording film 304 and a metal reflecting film 302, it is notpossible to enhance the ratio Ac/Aa while maintaining a large differencein reflectance between crystal and amorphous assuming that theabsorptivity of the recording film in a crystal state is Ac and theabsorptivity of the recording film in an amorphous state is Aa. However,as this embodiment, if three layers of dielectric film203/semitransparent metal film 204/dielectric film 205 are providedbetween the recording film 206 and the reflecting film 202 and ifanother semitransparent metal film 208 is provided on the light incidentside, it is possible to enhance Ac/Aa while maintaining the highreflectance change.

The internal energy of the recording film 206 in the crystal state islow, and the internal energy of the recording film in the amorphousstate is high. That is, in order to melt the recording film in thecrystal state, it is required to give a higher energy than that to meltthe recording film 206 in the amorphous state. This means that theamorphous portion melts prior to the crystal portion in an adjacentregion if both crystal and amorphous parts exist neighboring each otherin a laser spot. Therefore, when a direct overwrite operation is carriedout, a phenomenon takes place wherein although an amorphous mark meltsto be a new mark, the crystal portion around the amorphous mark does notmelt to form no mark. That is, the status before overwrite influencesthe state after overwrite. Therefore, new recording information ismodulated by the previous information, and the overwrite erasing ratiois insufficient.

One means for solving this is to enhance Ac/Aa. Usually, in a so-calledHigh-to-Low medium wherein the reflectance of the crystal portion Rc ishigher than the reflectance of the amorphous portion Ra, Ac/Aa<1,because Ac+Rc=Aa+Ra=1 in the medium having the total reflectionstructure. For example, if Rc−Ra=0.2, there is a difference of 0.2between Ac and Aa, so that it is required to provide a film forabsorbing light in order to decrease the difference. However, forexample, in order to achieve this on the conditions that Rc is 15% ormore and Ra is 3% or less, the distance between the recording film 206and the reflecting film 202 must be 25 nm or more. If a dielectricmaterial having a low heat conductivity, such as ZnS:SiO₂, is used inorder to decrease this distance, cooling after recording is too slow,and recrystallization and cross erase remarkably occur. In addition, ifa dielectric film having a high heat conductivity is used so as to beadjacent to the recording film, cooling occurs immediately after heatingthe recording film 206, so that the sensitivity of the recording mediumis remarkably bad. This can be improved to some extent if a dielectricfilm having the same heat conductivity as that of ZnS:SiO₂ is providedadjacent to the recording film 206 and if a dielectric film having ahigh heat conductivity is stacked thereon. However, since the heatconductivity is far worse than that in a case where a metal is used asthis embodiment, it is not possible to radiate heat in lateraldirections, and quenching is insufficient immediately after a recordingpower irradiation.

An example of this embodiment will be described below.

FIG. 9 shows a sectional construction of an example of an opticalrecording medium according to the present invention. The recordingmedium in this example has a construction wherein a reflecting film 202of an Ag alloy, a dielectric film 203 of ZnS:SiO₂, a semitransparentfilm 204 of an Ag alloy, a dielectric film 205 of ZnS:SiO₂, a recordingfilm 206 of GeSbTe, a dielectric film 207 of ZnS:SiO₂ and asemitransparent film 208 of an Ag alloy are stacked in order from theopposite side to light incidence. The thickness of these films is 100nm, 10 nm, 15 nm, 10 nm, 12 nm, 120 nm and 12 nm, respectively, in thesame order. A single layer film of an Ag alloy was separately prepared,and its complex refractive index (optical constant) was measured byellipsometry. As a result, the complex refractive index was 0.50-1.73iat a wavelength of 405 nm. In addition, the complex refractive index(optical constant) of GeSbTe forming the recording film was similarlymeasured. As a result, the complex refractive index was 2.6-2.1i in anamorphous state, and 1.45-2.8i in a crystal state. In addition, theabove described ZnS:SiO₂ is ZnS SiO₂=80:20 in mole ratio.

The phase-change optical recording media in this example was prepared bydepositing films on a polycarbonate substrate 101 having a thickness of1.1 mm in the above-described order by the sputtering method. Afterdeposition, an ultraviolet curing resin was spin-coated to bephoto-cured to form a cover layer 209 having a thickness of 0.1 mm.Then, the whole surface of the recording film was crystallized by aphase change medium initializing system having an elliptical beam havinga width of 50 μm and a length of 1 μm, to complete the medium. Grooveshaving a depth of 40 nm are provided on the substrate at intervals of0.6 μm. When it is evaluated as a land groove recording, a substratehaving a track pitch of 0.3 μm was used. It is hereinafter assumed thatthe groove track means a track farther from the light incident surface,and the land track means a track nearer to the light incident surface.

After the reflectance of a non-grooved portion having no groove in themedium was measured, it was 17.5% in an initialized portion and it was1.7% in an uninitialized portion.

Using the above described medium, record and erase experiments werecarried out. Evaluation was carried out by means of an optical diskevaluating system having a pick-up which has an objective lens ofnumerical aperture NA=0.85 and a semiconductor laser having a wavelengthof 405 nm. Assuming that a pattern having a shortest mark length of 0.15μm was 3T and a pattern having a longest mark length of 0.55 μm was 11T,a recording experiment was carried out by irradiating with pulsepatterns formed by combining a series of 3, 4, . . . , 11T at random.

The recording method was carried out by using the optimum recordingpower. When a random pattern was recorded only once, the jitter valuewas 8.2%. Then, the random pattern was overwritten on a groove track tentimes. As a result, the obtained jitter was 8.2%. Then, the same randompattern was recorded on adjacent land tracks on both sides ten times,respectively, to be returned to the original track to examine thevariation in jitter. As a result, the jitter was 8.2% which was notdeteriorated.

This result was obtained by providing a very thin semitransparent Agalloy film 208 on the light incident side and by carrying outabsorptivity correction between crystal and amorphous portions of therecording film 206. That is, if absorptivity correction is not carriedout, the ratio Ac/Aa of the absorptivity Ac of crystal to theabsorptivity of amorphous is ˜0.75. In the case of the optical recordingmedium in this example, the absorptivity is corrected to be 0.90, sothat it is difficult to cause a different in temperature between arecording mark and other portions. Therefore, there is not caused aphenomenon wherein new recording is modulated by the remaining lightduring overwrite, i.e. by the previous recording. In addition, since theabsolute value of light absorption of a recording mark (i.e. amorphous)in an adjacent track is suppressed, the mark deterioration in theadjacent track during cross erase is small.

As reflected therein, jitter values before and after cross erase areimproved. In particular, the increase of jitter values due to crosserase does not occur.

In this example, operation at a wavelength of 405 nm has been described.After various possibilities was studied, it was revealed that there ispreferably used a material wherein the real part n of refractive indexsatisfies 0<n<1 at a wavelength to be used. Therefore, in addition toAg, Al or its alloy can be used at the above-described wavelength.Specifically, assuming that the complex refractive index of thesemitransparent metal film 208 is n₂₀₈−ik₂₀₈, n₂₀₈ is preferably smallerif k₂₀₈ is the same, and k₂₀₈ is preferably larger if n₂₀₈ is the same.If another metal is added to pure Al or pure Ag, there is a tendency toincrease n₂₀₈ as the added amount increases. Therefore, from therelationship between the composition of the alloy and theabove-described refractive index, the composition ratio of Ag in the Agalloy is preferably 90%, and the composition ratio of Al in the Al alloyis preferably 90%. The reason for this is that, if they are beyond theseranges, the difference in reflectance between amorphous and crystalportions decreases to decrease regenerative signals, and theabsorptivity correcting effects are insufficient.

A medium was prepared by using an Ag alloy as the material of thesemitransparent metal film 208 and changing the composition ratio of Agin the Ag alloy. With this medium, the measured results of jitter valuesare shown in FIG. 17. As shown in FIG. 17, there were prepared a mediumin example 1 wherein the composition ratio of Ag in the semitransparentmetal film 208 is Ag=97.5 at %, a medium in example 2 wherein thecomposition ratio of Ag is Ag=92.0 at %, and a medium in comparativeexample wherein the composition ratio of Ag is Ag=88.0 at %. Withrespect to these media in examples 1, 2 and comparative example, jittervalues after ten random signal recording operations in only acorresponding track are shown in 1Tr column of FIG. 17, and jittervalues after ten overwrite operations of the same pattern in an adjacenttrack are shown in 3Tr column of FIG. 17. Furthermore, since the effectswere only examined when the material of the semitransparent metal film208 is changed, the same Ag alloy (Ag=97.5 at %, Pd=1.0 at %, Cu=1.5 at%) was used as the materials of reflecting film 202 and semitransparentmetal film 204. FIG. 17 also shows the complex refractive index measuredby light having a light source wavelength of 405 nm. As can be seen fromFIG. 17, in examples 1 and 2 wherein the composition rate of Ag is high,jitter values are lower than those in comparative example before andafter overwrite. Therefore, it is found that the composition ratio of Agin the Ag alloy is preferably 90% or more.

When a metal other than the above described Ag alloy is used as thematerial of the semitransparent film 208, e.g. when an Al alloy is used,the same effects can be obtained if the composition ratio of Al in theAl alloy is 90% or more. However, for example, at a wavelength of 405nm, if Au, Cu or another metal having a greater value of n₂₀₈ than thatof Ag and Al is used, preferred effects can not be obtained. Forexample, when Au was used, if the reflectance of the crystal portion was15%, the reflectance of the amorphous portion (uninitialized portion)was a large value of 6%, so that it was not possible to obtain asufficient signal strength. After a jitter value was measured, thejitter value was 9.5% when only one random pattern was recorded.Moreover, the jitter value was 12% after the mark row of the randompattern was overwritten ten times, and the jitter value was 14.5% afteroverwriting in an adjacent track. Thus, the difference in reflectance isnot only small, but the rise of the jitter value due to overwrite islarge. Therefore, it was revealed that the absorptivity can not besufficiently corrected, and the influence on the recording mark in theadjacent track can not be suppressed. The reason for this is that, sincethe optical constant of Au is 1.57-1.93i which is beyond the scope ofthis embodiment at the used wavelength, so that the ratio ofabsorptivity Ac/Aa can not be corrected.

However, when a light source having a longer wavelength range is used,e.g. a wavelength of about 650 nm, Au, Cu and their alloys can be usedin addition to Ag, Al and their alloys. In that case, the compositionratio of Au or Cu is preferably higher, e.g. 90% or more. Also, if an Auor Cu alloy is used as the material of the semitransparent film 204, thecomposition ratio of Au or Cu is preferably higher, e.g. 90% or more.

While nothing has been provided between the recording film 206 and thedielectric films 205 and 207 sandwiching the recording film 206therebetween in this example, a thin film may be provided before andafter the recording film 206 in order to improve the erasing rate. Thematerials of this thin film include GeN, SiC, SiN and CrO. If the heatconductivity of this thin film is within the range defined in thisembodiment, i.e. below 2.0 W/mK, it is within the scope of thisembodiment, so that the above-described thin film may be substituted fora part of the dielectric film 105 of FIG. 1. Even if the heatconductivity of the above described thin film exceeds 2.0 W/mK, if thethickness of the thin film is 5 nm or less, it is possible to obtain thesame effects as those in this embodiment, and the above described thinfilm can be substituted for a part of the dielectric film of FIG. 1.

The present invention should not be limited to the above describedexamples, but the invention can be modified without departing from theprinciple of the invention. For example, the recording film may beformed of any one of GeSbTeSn, GeSbTeBi, AgInSbTe, InSbTe, AgInGeSbTe,GeInSbTe and AgInSbTeV in addition to GeSbTe.

In the above described embodiment, the reflecting film 202, thedielectric film 203, the semitransparent metal film 204, the dielectricfilm 205, the recording film 206, the dielectric film 207 and thesemitransparent metal film 208 have been sequentially stacked on thesubstrate 201 so as to allow light to be incident on the semitransparentmetal film 208. Conversely, the semitransparent metal film 208, thedielectric film 207, the recording film 206, the dielectric film 205,the semitransparent metal film 204, the dielectric film 203 and thereflecting film 202 may be sequentially stacked on the substrate so asto allow light to be incident on the substrate. In this case, it isrequired to use an objective lens suited to allow light to be incidenton the substrate, as a light source. In addition, a protective film forprotecting the reflecting film 202 may be provided on a surface oppositeto a surface on which the dielectric film 203 is formed.

U.S. Pat. No. 4,839,861 (Ikegawa, S. et al.) discloses a techniquewherein an Au-Ge material is used as the material of a recording filmfor recording by a phase transition between crystal and crystal. Thus,the phase change optical recording should not be limited to a recordingwhich is strictly carried out by only the phase transition betweencrystal and amorphous, but it may be a phase transition between aplurality of different crystalline states, or a phase transition betweencrystal and amorphous or between microcrystal and bulky crystal. Inshort, if the atomic arrangement can be reversibly changed to change theoptical reflectance to hold recording, the present invention can beapplied.

As described above, according to the present invention, even if thetrack pitch is decreased, it is possible to prevent cross erase, so thatit is possible to enhance a recording capacity per one, therebyincreasing the amount of information to be recorded.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcepts as defined by the appended claims and their equivalents.

What is claimed is:
 1. An optical recording medium comprising: areflecting film; a first transparent film provided on the reflectingfilm; a first semitransparent film provided on the first transparentfilm; a second transparent film provided on the first semitransparentfilm; a recording film provided on the second transparent film, therecording film being capable of reversibly changing an atomicarrangement; and a third transparent film provided on the recordingfilm, wherein the first semitransparent film has a complex refractiveindex of n−ik satisfying relationships of 0<n<1 and 1<k, and a productof a thickness d (nm) of the first semitransparent film and anextinction coefficient k of the complex refractive index is d×k≦44. 2.An optical recording medium as set forth in claim 1, wherein a distancebetween the first semitransparent film and the recording film in adirection perpendicular to a film plane is 5 nm or longer but 15 nm orshorter.
 3. An optical recording medium as set forth in claim 1, whereinthe second transparent film has a heat conductivity of 2.0 W/mK or less.4. An optical recording medium as set forth in claim 1, wherein thereflecting film has a complex refractive index of n′−ik′ satisfying arelationship of 1<k′, and a product of a thickness r of the reflectingfilm and an extinction coefficient k′ of the complex refractive index isr×k′≧60.
 5. An optical recording medium as set forth in claim 1, whereinthe semitransparent film is formed of a simple substance or alloycontaining 90 at % or more of at least one of aluminum, silver, gold andcopper.
 6. An optical recording medium as set forth in claim 1, whichfurther comprises a second semitransparent film provided on the thirdtransparent film, the second semitransparent film being formed of asimple substance or alloy containing 90 at % or more of at least one ofaluminum, silver, gold and copper, the second semitransparent filmhaving a thickness of 25 nm or less.
 7. An optical recording mediumcomprising: a reflecting film; a first transparent film provided on thereflecting film; a first semitransparent film provided on the firsttransparent film; a second transparent film provided on the firstsemitransparent film; a recording film provided on the secondtransparent film, the recording film being capable of reversiblychanging an atomic arrangement; and a third transparent film provided onthe recording film, wherein the first semitransparent film is formed ofa simple substance or alloy containing 90 at % or more of at least oneof aluminum, silver, gold and copper, the first semitransparent filmhaving a thickness of 25 nm or less.
 8. An optical recording medium asset forth in claim 7, wherein a distance between the firstsemitransparent film and the recording film in a direction perpendicularto a film plane is 5 nm or longer but 15 nm or shorter.
 9. An opticalrecording medium as set forth in claim 7, wherein the second transparentfilm has a heat conductivity of 2.0 W/mK or less.
 10. An opticalrecording medium as set forth in claim 7, wherein the reflecting filmhas a complex refractive index of n−ik satisfying a relationship of 1<k,and a product of a thickness r of the reflecting film and an extinctioncoefficient k of the complex refractive index is r×k≧60.
 11. An opticalrecording medium as set forth in claim 7, which further comprises asecond semitransparent film provided on the third transparent film, thesecond semitransparent film being formed of a simple substance or alloycontaining 90 at % or more of at least one of aluminum, silver, gold andcopper, the second semitransparent film having a thickness of 25 nm orless.
 12. An optical recording medium comprising: a reflecting film; afirst transparent film provided on the reflecting film; a firstsemitransparent film provided on the first transparent film; a secondtransparent film provided on the first semitransparent film; a recordingfilm provided on the second transparent film, the recording film beingcapable of reversibly changing an atomic arrangement; a thirdtransparent film provided on the recording film; and a secondsemitransparent film provided on the third transparent film, wherein thefirst semitransparent film is formed of a simple substance or alloycontaining 90 at % or more of at least one of aluminum, silver, gold andcopper, the first semitransparent film having a thickness of 25 nm orless, and wherein the second semitransparent film is formed of a simplesubstance or alloy containing 90 at % or more of at least one ofaluminum, silver, gold and copper, the second semitransparent filmhaving a thickness of 25 nm or less.
 13. An optical recording medium asset forth in claim 12, wherein a distance between the firstsemitransparent film and the recording film in a direction perpendicularto a film plane is 5 nm or longer but 15 nm or shorter.
 14. An opticalrecording medium as set forth in claim 12, wherein the secondtransparent film has a heat conductivity of 2.0 W/mK or less.
 15. Anoptical recording medium as set forth in claim 12, wherein thereflecting film has a complex refractive index of n−ik satisfying arelationship of 1<k, and a product of a thickness r of the reflectingfilm and an extinction coefficient k of the complex refractive index isr×k≧60.
 16. An optical recording medium as set forth in claim 12,wherein the first semitransparent film has a complex refractive index ofn−ik satisfying relationships of 0<n<1 and 1<k.